1. Field of the Invention
The present invention relates to a photoelectric conversion device for outputting an output voltage according to an incident light.
2. Description of the Related Art
Presently, photoelectric conversion devices are used as image reading devices including facsimiles, image scanners, digital copying machines, and X-ray imaging devices. As the photoelectric conversion device, the contact image sensor (CIS), which is manufactured using single-crystal silicon chips, is widely known.
In other words, multiple photoelectric conversion devices are used as shown in FIG. 9. A photoelectric conversion device 41 includes a photoelectric conversion unit group 43 and a signal processing circuit 42. The photoelectric conversion unit group 43 includes multiple photoelectric conversion units (not shown) for each of pixels. The same reference voltage VREF is impressed on each of the photoelectric conversion devices 41 as shown in FIG. 10.
The photoelectric conversion unit outputs an output voltage VOUT according to a reset voltage Vreset used for resetting the photoelectric conversion unit and incident light to the signal processing circuit 42. The photoelectric conversion unit group 43 outputs the output voltages VOUT from the respective photoelectric conversion units in a time-series manner to the signal processing circuit 42. The signal processing circuit 42 outputs an output voltage VOUT2 according to the incident light by applying a predetermined process to the output voltage VOUT.
A description will now be given of operations of the photoelectric conversion unit group 43 and the signal processing circuit 42 in the conventional photoelectric conversion device 41. FIG. 11 shows a signal processing circuit of a conventional photoelectric conversion device.
When incident light enters photoelectric conversion means (not shown) mounted on the photoelectric conversion unit in the photoelectric conversion unit group 43, the photoelectric conversion means holds an optical charge. A reference signal upon a reset by the reset voltage, and an optical signal according to the quantity of the optical charge of the photoelectric conversion means are input to a sample-and-hold circuit 21 of the signal processing circuit 42. Multiple photoelectric conversion units are provided, and the reference signal and the optical signal output from the photoelectric conversion means of the respective photoelectric conversion units are input in a time-series manner into the sample-and-hold circuit 21. The sample-and-hold circuit 21, based on a signal φSH1, samples the reference signal input in a first half period, and holds the sampled reference signal in a last half period. In the last half period, a subtractor 24 obtains a difference between the sampled and held reference signal received via a buffer amplifier 22 and the optical signal received via a buffer amplifier 23 as described in JP 2005-012752 A (Patent Document 1).
However, according to the technology described in Patent Document 1, amplifiers on which the reference voltage VREF is impressed are not turned off completely, and currents thus always flow into a terminal on which the reference voltage VREF is impressed. As a result, when the reference voltage VREF is externally generated and supplied, the reference voltage VREF, which is externally generated, fluctuates according to the amount of the currents flowing into the terminal on which the reference voltage VREF is impressed. On this occasion, when the reference voltage VREF, which is externally generated, is impressed on the multiple photoelectric conversion devices 41, fluctuation of the reference voltage VREF in one photoelectric conversion device 41 causes the reference voltages VREF supplied for the other photoelectric conversion devices 41 to fluctuate, resulting in malfunction of the entire photoelectric conversion device 41. Therefore, there is required a solution to prevent the malfunction.